... is strange. I can nearly beat it, and I presume I eventually will. It would be a lot less frustrating, though, if the ghost didn't appear to boost straight through the gate towards the end which (ironically) seems to have a very rough and oversized collision box. If I go within about 20 car widths of the ghost I will hit an invisible wall and stop. So it's harder than it should be, to beat the ghost, as I can't fly directly through the same thing.

Assuming you do mean Dream Valley, yeah, the ghost there seems to ignore invisible boundaries. Stupid, but it's a stupid level so I expected nothing less. Basically, make sure that you're as far ahead as possible above the ghost before you enter the water part by absuing reckless boost/roll glitch. After you reach the water, make it your priority to NOT HIT ANYTHING, which is harder than it sounds due to Dream Valley being a total mess. Once you turn into a plane boost through the opening and you should be able to finish ahead of the ghost. Speedy drifter characters like Sonic and NiGHTS are probably the best to use.

If you're still working on it you can overtake the Sumo ghost big time in the water section. Assuming you are not doing the mirror version, when you first enter the water section you can trick upwards of 5 times depending how high your angle of entry is while going to the right. The drop after that you can trick 2-3 times depending on speed, and once you launch off the rock-ramp you basically barrel roll to victory.

When you do launch off that ramp don't worry about hitting the map's geometry since you will just clip through most of it anyway, just angle yourself into the LEFT path when falling. If you do it perfectly you will land at the lowest water level on top of the boost pad. It's still pretty much a coin flip for me in that section though, because sometimes I'll launch at an excellent angle and other times no matter how much I roll I will end up in a rock.

Take one of those console mods that gives full speed and boost. You should be able to just out race the ghost with that. Just practice a bit on the avoiding of impacts.

One thing I notice everyone does, they waste one lap and go around, then hit the boost right before the finish line, effectively starting the lap with at least level 3 boost. You immediately regain the boost after crossing and then can use it again wherever you need it.

It's sorta like getting an extra free boost. (Sorta why I feel an item boost shouldn't have been in the Time Trials, but that's for another thread. :P)

I'm guessing you're already doing this, but risk-boosting like a maniac in the flying segments is a must. Dream Valley is one of the best tracks in the game to chain together risk boosts, and doing so is immensely helpful.

For that one, I've found that the best trick is not to attempt any stunts on the last jump, and instead try to line yourself up in the air to start drifting into the curve right when you land. You can then switch-drift and take the corner with surprising ease. Works more often than not for me. :)